A limitation in fields such as biochemical sampling and analysis is the ability to precisely control uptake and/or dispersion of small quantities of fluid samples. For example, mass spectrometry (MS) is a common technique used in the analysis of biological samples such as proteins, where precisely controlled dispersion is required. Dispersion of the sample through an emitter using electrospray ionization (ESI) limits the degree of fragmentation compared to other methods such as electron ionization (EI) or chemical ionization (CI). As the flow rate drops, charged droplets that are ejected from the electrospray plume become smaller and the efficiency of charge transfer to analyte molecules in solution improves. Moving to flow rates below about 1000 nL/min, the so-called nano-electrospray (nanoESI) regime is reached leading to best ionization efficiency and the elimination of charge competition between analytes. The architecture of the emitter is important for generating stable electrospray, where cone-jet mode (a Taylor cone) is supported by a range of tip diameters at a given flow rate and applied voltage, with finer tips having smaller diameters being useful for lower flow rates. Popular commercial emitters for these lower flow rates are fused-silica capillaries that have been pulled to a fine, tapered exit. These single-channel tapered emitters are limited in the range of flow rates they can use for nanoESI, and suffer from clogging when the diameter gets small (<15 μm diameter). Furthermore, flow rates below ˜100 nL/min are difficult to control, and are not compatible with upstream fluidic applications such as nano-liquid chromatography (nano-LC).
One way to take advantage of the benefits of low-flow nanoESI while using higher flow rates is to split the input flow into many smaller flows, each generating its own Taylor cone independent of one another. In this way, the effects of small charged droplets are still observed but the input flow is more reasonable and all of the analyte solution reaches the MS. The theoretical relationship between electrospray current (Itotal) at a given flow rate and the electrospray current (Is) of the same flow split into n individual emitters has been found to be Itotal=√{square root over (n)}Is, meaning that the detection signal can be enhanced by a factor of √{square root over (n)} by having an emitter with n separate spraying tips. A variety of emitters have been developed using this approach, including examples fabricated by microchip fabrication techniques, laser ablation, or simply assembling an array of conventional fused-silica capillary-based emitters. Drawbacks of such multiple electrospray (MES) emitters are that they are typically large and do not couple well with traditional MS inlets, and/or they are complicated to fabricate.
Microstructured fibres (MSFs) present an alternative approach to MES emitters, as they essentially comprise a series of channels in a single fibre having dimensions compatible with conventional LC and MS equipment. These fibres are used in the field of photonics, where the channels are part of a cladding designed to guide light through the core by total internal reflectance. MSFs have also been demonstrated as sensors, using the channels to introduce analytes and measuring changes in the light propagated in the fibre.
The construction of MSFs starts with a preform at a manageable scale, either a disc through which holes are drilled or, more often, an assembly of tubes and rods, where the placement of the holes/tubes is pre-determined. This preform is then drawn, sometimes in steps, at high temperatures to a thin fibre that retains the pattern of the preform. In commercial MSFs, the channels are arranged to provide the best light-guiding performance, and typically there is a dense array of evenly-spaced homogeneous channels surrounding the core, usually in a circular or hexagonal pattern, with a pitch (distance between adjacent channels) on the order of ˜10-15 μm. Although commercially-available MSFs have been used as nanoESI emitters, MES was not observed as there is no barrier to flow from adjacent channels coalescing into a single spray. Limitations on commercially available MSFs, arising from their intended use in photonic applications which do not require channels spaced farther apart, make it difficult or impossible to achieve MES behaviour in emitter applications. Modifications, such as formation of polymer nozzles at the emitter tip in each channel, can provide MES behaviour under a certain conditions. However, the polymer nozzles complicate the fabrication process, are fragile, and may not be compatible with all analytes.